Measurement of semi-inclusive π+ electroproduction off the proton

<p> Semi-inclusive &pi;+ electroproduction on protons has been measured with the CLAS detector at Jefferson Lab. The measurement was performed on a liquid-hydrogen target using a 5.75 GeV electron beam. The complete five-fold differential cross sections were measured over a wide kinematic range including the complete range of azimuthal angles between hadronic and leptonic planes, &varphi;, enabling us to separate the &varphi;-dependent terms. Our measurements of the &varphi;-independent term of the cross section at low Bjorken x were found to be in fairly good agreement with pQCD calculations. Indeed, the conventional current fragmentation calculation can account for almost all of the observed cross section, even at small &pi;+ momentum. The measured center-of-momentum spectra are in qualitative agreement with high-energy data, which suggests a surprising numerical similarity between the spectator diquark fragmentation in the present reaction and the antiquark fragmentation measured in e+e&minus; collisions. We have observed that the two &varphi;-dependent terms of the cross section are small. Within our precision the cos2&varphi; term is compatible with zero, except for the low-z region, and the measured cos&varphi; term is much smaller in magnitude than the sum of the Cahn and Berger effects.</p>